Gut-protective effects of dietary niacin in juvenile turbot (Scophthalmus maximus L.) fed a high-lipid diet: modulation of mucosal immune response, barrier function, and gut microbiota

This study investigated the protective effects of dietary niacin on the intestinal health of juvenile turbot fed a high-lipid diet (HLD). Two isonitrogenous and isolipidic diets were formulated, including a HLD without niacin addition (HL0) and a HLD supplemented with 80 mg/kg niacin (HL80). Turbot (approximately 13.2 g) were fed two experimental diets for 10 weeks, with each diet assigned to triplicate tanks. Results showed that niacin significantly improved the weight gain rate and specific growth rate of turbot. Dietary niacin significantly down-regulated the expression of toll-like receptors (TLRs), along with their associated signaling components, including myeloid differentiation factor 88 (MyD88), interferon regulatory factor-7 (IRF-7), p38 mitogen-activated protein kinase (p38MAPK), and activator protein-1 (AP-1). This reduction also extended to key molecules in the downstream nuclear factor-kappa B (NF-κB) signaling pathway, such as NF-κB, IκB kinase β (IKKβ), and IκB kinase γ (IKKγ) (P < 0.05). Similarly, niacin supplementation in HLD markedly decreased the gene expression of pro-inflammatory cytokines and the signaling molecule myosin light chain kinase (MLCK), while increasing the expression of anti-inflammatory cytokines, antibacterial peptides, and barrier-enhancing proteins (P < 0.05). Additionally, HLD-induced intestinal oxidative stress was mitigated by niacin supplementation, as demonstrated by the significant upregulation of nuclear factor E2-related factor 2 (Nrf2), glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), and heme oxygenase-1 (HO-1) (P < 0.05). Compared with the HL0 group, dietary niacin significantly increased the abundance of short-chain fatty acids (SCFAs)-producing bacteria (Bacteroides, Faecalibacterium, Roseburia, etc.) and lactic acid bacteria (Lactobacillus, Streptococcus, Lactococcus, etc.). In conclusion, these results indicated that dietary niacin could enhance intestinal mucosal immunity, physical barrier function, and antioxidant capacity through coordinated regulation of TLR-MyD88-NF-κB, MLCK, and Nrf2 signaling pathways. Combined with microbial community optimization, this collectively alleviates the negative effects of high-lipid diets on the intestinal health of turbot.